124 VOLCANIC BOCKS. 



each other, in the act of cooling. This gentleman fused 

 seven hundred weight of Dudley basalt, called Rowley rag- 

 stone, maintaining the fire for six hours, and allowing it to 

 cool so slowly, that eight days elapsed before it was removed 

 from the furnace. The mass was wedge-shaped, four feet 

 and a half long, two feet and a half wide, eighteen inches 

 thick at one end, and four at the other, an inequality of 

 form admirably adapted for exhibiting the different rates of 

 cooling, and the various kinds of texture thus induced. 

 "Where the mass was thinnest, and the cooling most rapid, 

 the texture was vitreous or glassy ; where it was thickest, 

 and the cooling slowest, it was stony, while the intermediate 

 parts exhibited a transition state. Numerous spheroids 

 had been formed; where two came in contact, they were 

 compressed, and when several met they formed prisms. 

 "We may form some idea of the arrangement, if we conceive 

 a number of cannon-balls to be piled on each other, and 

 reduced to a fluid condition ; the pressure of these globular 

 bodies on each other would produce a columnar arrange- 

 ment, similar to that observable in basalt. 



Basalt is frequently vesicular, and contains small cavities, 

 caused by the escape of gases while the rock was in a fused 

 condition. These have been subsequently filled with infil- 

 trations of carbonate of lime, agates, zeolite, and other 

 minerals. As these nodules have in general an elongated 

 shape, similar to an almond, such basalts are termed amyg- 

 daloidal. Basalt, like porphyry, has been ejected, in many 

 instances, from fissures in the bed of the ocean, and flowed 

 over the strata there deposited. The chief British localities 

 for these rocks are Durham and Dudley, the Highlands 

 of Scotland, and Edinburgh. 



VOLCANIC ROCKS. Volcanic productions are of so com- 

 pound a nature, and exhibit varieties of combination so 

 numerous and minute, as to require a separate treatise for 

 their description. The reader is, therefore, referred to the 

 works of Humboldt,* Dr. Daubeny, or Mr. Poulett Scrope;f 

 they are usually divided in trachytic and basaltic lavas. 

 The former are of a light earthy colour, and feldspar is 

 usually an essential element in their composition. The 



* Humboldt's Cosmos : Bohn's edition, vol. i. p. 225. f On Vblcanoes. 



